Cryo-EM structure of the agonist-bound Hsp90-XAP2-AHR cytosolic complex

Summary Living organisms have developed protein sensors helping them to adapt to their environment[1][1]. The aryl hydrocarbon receptor (AHR) is an emblematic member of this class of proteins, and a ligand-dependent transcription factor that mediates a broad spectrum of (patho)physiological processes in response to numerous substances including pollutants, natural products and metabolites[2][2]. However, in the absence of high-resolution structural data, a molecular understanding of how AHR is activated by such diverse compounds is lacking. Here we present a 2.85 Å cryo-electron microscopy structure of the cytosolic complex comprising AHR bound to the ligand indirubin, the chaperone Hsp90 and the co-chaperone XAP2. The structure reveals a closed Hsp90 dimer with AHR threaded through its lumen. XAP2 directly interacts with Hsp90 and the AHR ligand-binding domain, thereby acting as a brace stabilizing the entire complex. Importantly, we provide the first experimental visualization of the AHR PAS-B domain bound to a ligand, revealing a unique organization of the ligand-binding pocket and the structural determinants of ligand-binding specificity and promiscuity of the receptor. By providing unprecedented structural details of the molecular initiating event leading to AHR activation, our study rationalizes prior biochemical data and provides a framework for future mechanistic studies and structure-guided drug design. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-2